MG Horde is active, and tends to be guild based in terms of activity. A common issue is that many players roll on MG Horde expecting MG Alliance levels of activity in Stormwind to be found in Org, however a lot of MG Horde rp guilds do not base themselves in Org. Org just isnt a great hub for all the different races of the Horde, nor is it anywhere near as immersive as Stormwind.
You can find various MG Horde rp guilds advertising on the server forums.
I also recommend the Horde Vanguard for Horde faction rp, which has some upcoming rp campaigns about to be announced.